The present invention relates to a semiconductor pressure transducer and in particular to a semiconductor pressure transducer which is stable under exterior inductive noise or pollution.
A semiconductor pressure transducer for converting variation of pressure, differential pressure or the like into that of a resistance value, obtained by forming a strain gauge resistor element on a substrate of semiconductor monocrystal such as silicon, is known. Within the silicon monocrystal substrate acting as a strain causing body included in such a semiconductor pressure transducer, a strain gauge resistor element having a conductivity which is opposite to that of the substrate is selectively and integrally formed. The strain gauge resistor element is covered with silicon dioxide film for the purpose of passivation and insulation. In addition, an electrode of metal such as aluminium is provided to feed the electrical output of the strain gaguge resistor element to the outside.
In a semiconductor pressure transducer of such configuration, the movement of ions such as ions of sodium attached or deposited onto the surface of the silicon dioxide film which covers the strain gauge resistor element or the movement of ions within the silicon dioxide film varies the potential of the silicon dioxide film. This results in the drawback that the output of the transducer is varied according to a change of temperature or environment and time lapse.
As described in Japanese Patent Application Laid-Open No. 33092/80, it is proposed to form a conductive film such as a gold film on the silicon dioxide film which covers the strain gauge resistor element. In such configuration, the movable ions on the surface of the silicon dioxide film and within the silicon dioxide film are fixed by the conductive film. Accordingly, the above described drift problem of the transducer output with time is resolved. In this case, however, a new different problem involving the performance characteristic is produced. Namely, the output, shifted by the temperature variation, does not return to its initial state even if the temperature is restored to its initial value. In other words, hysteresis with temperature change occurs. This temperature hysteresis phenomenon is based upon the difference between the heat expansion rate of the conductive film such as gold film and that of the silicon monocrystal substrate. The conductive film with a large heat expansion rate breaks down on account of the temperature variation. Accordingly, a remnant strain occurs in the strain gauge resistor element which is just under the conductive film, causing the temperature hysteresis phenomenon.